A proposed structure of an internal combustion engine system includes an engine equipped with a port fuel injection valve arranged to inject a fuel into an intake port and with an in-cylinder fuel injection valve arranged to directly inject the fuel into each cylinder (see, for example, Patent Document 1). This proposed internal combustion engine system successively performs learning with regard to the air-fuel ratio of the engine with fuel injection from only one of the port fuel injection valve and the in-cylinder fuel injection valve and learning with regard to the air-fuel ratio with fuel injection from only the other of the port fuel injection valve and the in-cylinder fuel injection valve. This technique ensures the adequate learning with regard to the air-fuel ratios of both the port fuel injection valve and the in-cylinder fuel injection valve.    Patent Document 1: Japanese Patent Laid-Open No. 2005-330939